1. Field of the Invention
The present invention relates generally to cryogenic liquids and gases. In particular the present invention relates to the production and injection of a sterile cryogenic liquid in a manner which safeguards the sterility of the cryogenic liquid until it is dispensed.
2. Related Art
Cryogenic liquids and gases of high purity, such as nitrogen, helium, argon and the like, are preferred for a variety of industries. The semiconductor industry, for example, prefers ever increasingly purer nitrogen as a carrier gas for reactive species in the production of semiconductors.
Another industry desiring high purity inert gases is the food packaging industry, more particularly the canned food industry. Aluminum cans are being increasingly used in retail sales of soft drinks, fruit juices, coffee beverages, and the like. Aluminum cans, which can be made in two pieces, are often less expensive to produce than competing three piece cans. However, aluminum cans often do not have the compressive strength of steel cans, making the stacking of aluminum-canned food higher than a minimal height very difficult: the cans at the bottom of the stack are likely to fail. This is of course undesirable for a variety of reasons. Failure of cans could cause the food or beverage to leak from the can, causing slip-and-fall hazards for workers or store customers, and the spillage of food is often distasteful in terms of eye appeal, or because of foul smell. Therefore, if aluminum cans are to be used, it is desirous to use cryogenic liquids in droplet form dispensed into the can just before and/or during the sealing procedure. The cryogenic liquid vaporizes via heat transfer from the surroundings, and expands and thus pressurizes the can.
The use of inert, sterile cryogenic liquids in aluminum canning is known, and methods and apparatus are described in U.S. Pat. Nos. 4,620,962 and 4,759,848. the '962 patent is directed to a method and apparatus to make sterile cryogenic liquid, wherein the cryogenic liquid to be sterilized is first vaporized, then sterilized as a gas and finally reliquified to provide sterile cryogenic liquid. Vaporization is carried out in a heat exchanger which is also used to cool the sterilized gas. The '848 patent discloses a method and an apparatus to sterilize a cryogenic liquid, wherein the unsterilized cryogenic liquid is provided in liquid (saturated) state to a microporous filter submerged in a subcooling cryogenic liquid having a temperature cooler than the saturated stream, the cryogenic liquid being thus sterilized and subcooled. The purpose of subcooling is to prevent the formation of gas bubbles in the saturated stream and, therefore, assures that the saturated liquid can pass quickly through the pores of the filter. These methods and apparatus, while producing sterile cryogenic liquids and gases, do not disclose or suggest means for maintaining the sterility of the sterile cryogenic liquid.
A problem that still presents itself to the user of sterile cryogenic liquids is the fact that the bulk cryogenic liquid supply is not sterile. Further, even if a supply of sterile nitrogen were available, in many instances a metering device, such as a needle valve or metering pump, is used to dispense the sterile liquid. These metering devices typically have very small outlet nozzles which can easily become clogged with ice formed from water vapor in the surrounding air as the drops of liquid cryogen are dispensed into the can. Ambient air is, of course, not sterile, and thus contamination of the food or semiconductor or other product is an acute problem. It is of course possible in theory to provide a "clean room" for the dispensing area, where the atmosphere is sterile, but this may be cost prohibitive when large food canning lines or other production lines are involved.
There is therefore an unmet need for a method and apparatus which simply and efficiently produces sterile cryogenic liquids from the bulk raw supply, ensures the sterility of the cryogenic liquid, particularly under a variety of canning line conditions, and manages the injection of the sterile liquid cryogen into food products.